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On Earth, exposed olivine weathers and breaks down quickly because of the planet's relatively warm, wet weather.

Conditions are very different on Mars, which is cold and dry. How much of the unweathered mineral is found on Mars may provide clues about the planet's ancient climate.

If the mineral has been there since early in the planet's history, it would mean the planet has been cold and dry for most of its life.

But many scientists believe Mars was once much more like the Earth, with running water and a thicker atmosphere.

A team of United States researchers led by Todd Hoefen of the US Geological Survey in Denver, Colorado, reported the findings in the journal Science.

Nili Fossae is thought to have formed at least 3.6 billion years ago, giving an upper limit for the appearance of olivine on the surface.

The scientists wrote: "If the olivine was exposed shortly after the impact event, the martian surface may have been dry and cold for more than three billion years, but if the olivine was recently uncovered . . . it could have been cold and dry for as little as a few thousand years."